Get a peice of vacuum hose about three feet long of fairly large diameter. Take the hose and put one end to your ear and the other end down where the hose for the brake booster hooks to the intake manifold. With the engine running try and see if you can hear air rushing or maybe better air escaping. You may want to take the hose off and inspect it, if you hear air moving after doing what I just discribed. Vacuum leaks are tough to pinpoint sometimes,but they sure will make a difference in the way your car performs after their fixed. Stick with it you'll have brakes again soon.
If you find that there isn't a vacuum leak down around the hook up I mentioned, have someone step on the brake peddel while it's running and see if you can tell with the hose to your ear where the leak is coming from.
If it turns out the booster is just bad,I would try a used booster from a pick and pull before going for a new unit from the dealer, very rare for a booster in one of these cars to go out so young in years.
Hope that helps
Poolman